In telecommunication, a log-periodic antenna (LP, also known as a log-periodic array) is a broadband, multielement, unidirectional, narrow-beam antenna that has impedance and radiation characteristics that are regularly repetitive as a logarithmic function of the excitation frequency. The individual components are often dipoles, as in a log-periodic dipole array (LPDA). Log periodic antennas are designed to be self-similar and thus are fractal antenna arrays.
It is normal to drive alternating elements with a 180o (π radian) phase shift from the last element. This is normally done by wiring the elements alternatingly to the two wires in a balanced transmission line.
The length and spacing of the elements of a log-periodic antenna increase logarithmically from one end to the other.
The result of this structural condition is that if a plot is made of the input impedance as a function of log of frequency then the variation will be periodic i.e. the impedance will go through the cycles of variation in such a way that each cycle is exactly like its preceding one and hence the name.
This is a beam antenna which for a given boom length gives a higher gain than a Yagi antenna, it is a log-periodic where only two elements are driven with a series of parasitic elements (directors) in front of the smaller of the two driven elements.
The driven elements are folded dipoles which are linked by a length of balanced twin-lead feed line. The polar plot of the ZL special and the HB9CV is a heart shape (cardioid) while the typical Yagi antenna has a large forward lobe and a smaller back lobe.